The following description is provided solely to assist the understanding of the reader. None of the references cited or information provided is admitted to be prior art to the present invention. Each of the references cited herein is incorporated by reference in its entirety, to the same extent as if each reference were individually indicated to be incorporated by reference herein in its entirety.
The peroxisome proliferator-activated receptors (PPARs) form a subfamily in the nuclear receptor superfamily. Three isoforms, encoded by separate genes, have been identified thus far: PPARγ, PPARα, and PPARδ.
There are two PPARγ isoforms expressed at the protein level in mouse and human, γ1 and γ2. They differ only in that the latter has 30 additional amino acids at its N terminus due to differential promoter usage within the same gene, and subsequent alternative RNA processing. PPARγ2 is expressed primarily in adipose tissue, while PPARγ1 is expressed in a broad range of tissues.
Murine PPARα was the first member of this nuclear receptor subclass to be cloned; it has since been cloned from humans. PPARα is expressed in numerous metabolically active tissues, including liver, kidney, heart, skeletal muscle, and brown fat. It is also present in monocytes, vascular endothelium, and vascular smooth muscle cells. Activation of PPARα induces hepatic peroxisome proliferation, hepatomegaly, and hepatocarcinogenesis in rodents. These toxic effects are not observed in humans, although the same compounds activate PPARα across species.
Human PPARδ was cloned in the early 1990s and subsequently cloned from rodents. PPARδ is expressed in a wide range of tissues and cells; with the highest levels of expression found in the digestive tract, heart, kidney, liver, adipose, and brain.
The PPARs are ligand-dependent transcription factors that regulate target gene expression by binding to specific peroxisome proliferator response elements (PPREs) in enhancer sites of regulated genes. PPARs possess a modular structure composed of functional domains that include a DNA binding domain (DBD) and a ligand binding domain (LBD). The DBD specifically binds PPREs in the regulatory region of PPAR-responsive genes. The DBD, located in the C-terminal half of the receptor, contains the ligand-dependent activation domain, AF-2. Each receptor binds to its PPRE as a heterodimer with a retinoid X receptor (RXR). Upon binding an agonist, the conformation of a PPAR is altered and stabilized such that a binding cleft, made up in part of the AF-2 domain, is created and recruitment of transcriptional coactivators occurs. Coactivators augment the ability of nuclear receptors to initiate the transcription process. The result of the agonist-induced PPAR-coactivator interaction at the PPRE is an increase in gene transcription. Downregulation of gene expression by PPARs appears to occur through indirect mechanisms. (Bergen, et al., Diabetes Tech. & Ther., 2002, 4:163-174).
The first cloning of a PPAR (PPARα) occurred in the course of the search for the molecular target of rodent hepatic peroxisome proliferating agents. Since then, numerous fatty acids and their derivatives, including a variety of eicosanoids and prostaglandins, have been shown to serve as ligands of the PPARs. Thus, these receptors may play a central role in the sensing of nutrient levels and in the modulation of their metabolism. In addition, PPARs are the primary targets of selected classes of synthetic compounds that have been used in the successful treatment of diabetes and dyslipidemia. As such, an understanding of the molecular and physiological characteristics of these receptors has become extremely important to the development and utilization of drugs used to treat metabolic disorders.
Kota, et al., Pharmacological Research, 2005, 51:85-94, provides a review of biological mechanisms involving PPARs that includes a discussion of the possibility of using PPAR modulators for treating a variety of conditions, including chronic inflammatory disorders such as atherosclerosis, arthritis and inflammatory bowel syndrome, retinal disorders associated with angiogenesis, increased fertility, and neurodegenerative diseases.
Yousef, et al., Journal of Biomedicine and Biotechnology, 2004(3):156-166, discusses the anti-inflammatory effects of PPARα, PPARγ and PPARδ agonists, suggesting that PPAR agonists may have a role in treating neuronal diseases such as Alzheimer's disease, and autoimmune diseases such as inflammatory bowel disease and multiple sclerosis. A potential role for PPAR agonists in the treatment of Alzheimer's disease has been described in Combs, et al., Journal of Neuroscience 2000, 20(2):558, and such a role for PPAR agonists in Parkinson's disease is discussed in Breidert, et al., Journal of Neurochemistry, 2002, 82:615. A potential related function of PPAR agonists in treatment of Alzheimer's disease, that of regulation of the APP-processing enzyme BACE, has been discussed in Sastre, et al., Journal of Neuroscience, 2003, 23(30):9796. These studies collectively indicate PPAR agonists may provide advantages in treating a variety of neurodegenerative diseases by acting through complementary mechanisms.
Discussion of the anti-inflammatory effects of PPAR agonists is also available in Feinstein, Drug Discovery Today: Therapeutic Strategies, 2004, 1(1):29-34, in relation to multiple sclerosis and Alzheimer's disease; Patel, et al., Journal of Immunology, 2003, 170:2663-2669 in relation to chronic obstructive pulmonary disease and asthma (COPD); Lovett-Racke, et al., Journal of Immunology, 2004, 172:5790-5798 in relation to autoimmune disease; Malhotra, et al., Expert Opinions in Pharmacotherapy, 2005, 6(9):1455-1461, in relation to psoriasis; and Storer, et al., Journal of Neuroimmunology, 2005, 161:113-122, in relation to multiple sclerosis.
This wide range of roles for the PPARs that have been discovered suggest that PPARα, PPARγ and PPARδ may play a role in a wide range of events involving the vasculature, including atherosclerotic plaque formation and stability, thrombosis, vascular tone, angiogenesis, cancer, pregnancy, pulmonary disease, autoimmune disease, and neurological disorders.
Among the synthetic ligands identified for PPARs are thiazolidinediones (TZDs). These compounds were originally developed on the basis of their insulin-sensitizing effects in animal pharmacology studies. Subsequently, it was found that TZDs induced adipocyte differentiation and increased expression of adipocyte genes, including the adipocyte fatty acid-binding protein aP2. Independently, it was discovered that PPARγ interacted with a regulatory element of the aP2 gene that controlled its adipocyte-specific expression. On the basis of these seminal observations, experiments were preformed that determined that TZDs were PPARγ ligands and agonists and demonstrate a definite correlation between their in vitro PPARγ activities and their in vivo insulin-sensitizing actions. (Bergen, et al., supra).
Several TZDs, including troglitazone, rosiglitazone, and pioglitazone, have insulin-sensitizing and anti-diabetic activity in humans with type 2 diabetes and impaired glucose tolerance. Farglitazar is a very potent non-TZD PPAR-γ-selective agonist that was recently shown to have anti-diabetic as well as lipid-altering efficacy in humans. In addition to these potent PPARγ ligands, a subset of the non-steroidal anti-inflammatory drugs (NSAIDs), including indomethacin, fenoprofen, and ibuprofen, have displayed weak PPARγ and PPARα activities. (Bergen, et al., supra).
The fibrates, amphipathic carboxylic acids that have been proven useful in the treatment of hypertriglyceridemia, are PPARα ligands. The prototypical member of this compound class, clofibrate, was developed prior to the identification of PPARs, using in vivo assays in rodents to assess lipid-lowering efficacy. (Bergen, et al., supra).
Fu et al., Nature, 2003, 425:9093, demonstrated that the PPARα binding compound, oleylethanolamide, produces satiety and reduces body weight gain in mice.
Clofibrate and fenofibrate have been shown to activate PPARα with a 10-fold selectivity over PPARγ. Bezafibrate acts as a pan-agonist that shows similar potency on all three PPAR isoforms. Wy-14643, the 2-arylthioacetic acid analogue of clofibrate, is a potent murine PPARα agonist as well as a weak PPARγ agonist. In humans, all of the fibrates must be used at high doses (200-1,200 mg/day) to achieve efficacious lipid-lowering activity.
TZDs and non-TZDs have also been identified that are dual PPARγ/α agonists. By virtue of the additional PPARα agonist activity, this class of compounds has potent lipid-altering efficacy in addition to anti-hyperglycemic activity in animal models of diabetes and lipid disorders. KRP-297 is an example of a TZD dual PPARγ/α agonist (Fajas, J. Biol. Chem., 1997, 272:18779-18789); furthermore, DRF-2725 and AZ-242 are non-TZD dual PPARγ/α agonists. (Lohray, et al., J. Med. Chem., 2001, 44:2675-2678; Cronet, et al., Structure (Camb.), 2001, 9:699-706).
In order to define the physiological role of PPARδ, efforts have been made to develop novel compounds that activate this receptor in a selective manner. Amongst the α-substituted carboxylic acids previously described, the potent PPARδ ligand L-165041 demonstrated approximately 30-fold agonist selectivity for this receptor over PPARγ, and it was inactive on murine PPARα (Liebowitz, et al., 2000, FEBS Lett., 473:333-336). This compound was found to increase high-density lipoprotein levels in rodents. It was also reported that GW501516 was a potent, highly-selective PPARδ agonist that produced beneficial changes in serum lipid parameters in obese, insulin-resistant rhesus monkeys. (Oliver et al., Proc. Natl. Acad. Sci., 2001, 98:5306-5311).
In addition to the compounds discussed above, certain thiazole derivatives active on PPARs have been described. (Cadilla, et al., Internat. Appl. PCT/US01/149320, Internat. Publ. WO 02/062774, incorporated herein by reference in its entirety.)
Some tricyclic-α-alkyloxyphenylpropionic acids have been described as dual PPARα/γ agonists in Sauerberg, et al., J. Med. Chem. 2002, 45:789-804.
A group of compounds that are stated to have equal activity on PPARα/γ/δ is described in Morgensen, et al., Bioorg. & Med. Chem. Lett., 2002, 13:257-260.
Oliver et al., describes a selective PPARδ agonist that promotes reverse cholesterol transport. (Oliver, et al., supra)
Yamamoto et al., U.S. Pat. No. 3,489,767 describes “1-(phenylsulfonyl)-indolyl aliphatic acid derivatives” that are stated to have “antiphlogistic, analgesic and antipyretic actions.” (Col. 1, lines 16-19.)
Kato, et al., European patent application 94101551.3, Publication No. 0 610 793 A1, describes the use of 3-(5-methoxy-1-p-toluenesulfonylindol-3-yl)propionic acid (page 6) and 1-(2,3,6-triisopropylphenylsulfonyl)-indole-3-propionic acid (page 9) as intermediates in the synthesis of particular tetracyclic morpholine derivatives useful as analgesics.